Many advanced semiconductor fabrication processes involve the construction of high aspect ratio trenches on the micrometer or sub-micrometer scale. For example, micro-electro-mechanical systems (MEMS), which often contain moving parts a few microns wide, are characterized by deep vertical etching processes. The product contains three-dimensional structures with regions of deep, narrow trenches with near-vertical sidewalls. A typical example is a trench etched 5 microns wide by 100 microns deep. MEMS devices with these characteristics include sensors, actuators, and RF devices such as inductors and comb switches. All of these devices characteristically require deep vertical etching processes to separate moving mechanical parts, and finger-like features are very common. Manufacturers of MEMS devices do not currently have an accurate and inexpensive method to non-destructively measure the depth of etched high aspect ratio trenches. They need to have precise control over etch depth to produce a working device, and the measurement of etch depth is very important for process development and control. Current metrology technology cannot measure the depth of high aspect ratio trenches with speed and accuracy. Thus, the development of a non-contact metrology instrument that quickly and accurately measures the etched depth of high aspect ratio trenches, such as those formed by narrow finger-like structures, would greatly benefit MEMS manufacturers in process development and control.
Because of the very steep sidewalls inherent in such a structure, profiling instruments that use a stylus or other method of contact cannot accommodate an aspect ratio or lateral dimension of this nature. For example, atomic force microscopes (AFM) and stylus profilers are not suitable because even if the tip could penetrate the trench, it would not be able to follow the side wall, and the tip would break when exiting the trench. Standard non-contact optical instruments for measuring surface height are confocal microscopes, white light interferometers, phase shift interferometers, and triangulation techniques. Standard confocal microscopes fail because they confuse the signal from the top of the trench with the signal from the bottom when the trench is too narrow. When the width of the trench approaches the size of the source pinhole, as much or more light will be detected when the focus is on top of the trench as when it is at the bottom. Thus, a confusing signal is generated even when the bottom of the trench is far away from the focal plane. Confocal microscopes are also very slow since they require scanning the measurement sample longitudinally to find the plane of best focus. White light interferometers have similar difficulties in that they are slow and must scan longitudinally. In addition, the fringe signal is weak due to the light scattered from the walls and the top. Phase shift interferometers fail outright because phase unwrapping fails at steep sidewalls. Finally, triangulation techniques can only succeed if precise control of the direction of the incident beam relative to the direction of the trench is maintained so that the light can get into the trench from the side. This constraint makes such an instrument unfeasible.
The prior art is extensive for chromatic confocal sensors. In “Focus-Wavelength Encoded Optical Profilometer,” Optics Communications, Vol. 49, No. 4, p. 229, Mar. 15, 1984 by Molesini, et. al, a chromatic confocal system where the dispersive lens is a piano-convex singlet is described. The singlet forms a focused spot which is reimaged onto the object by a microscope objective. This system has the disadvantage that the microscope objective is used with a short, finite conjugate. Most microscope objectives are designed for long conjugates of 180 mm, 200 mm, or infinite. Thus, Molesini's system does not allow simultaneous imaging of the object and height profiling of the object. In addition, this system is not truly confocal, since it lacks an aperture in an image plane for the reflected light.
Chromatic confocal systems that provide an image by utilizing a rotating perforated disc are described by Scheruebl et. al. in U.S. Pat. No. 6,674,562, and in Saito et al in U.S. Pat. No. 6,188,514. A spectrometer is used in Scheruebl's system to quantitatively measure the height at a single point, and the measurement is then used as feedback for autofocus. Saito's system does not use a spectrometer, and the height-color information is only for subjective viewing. Both of these systems use the same source for image formation and for the height measurement. Since the single source is spread over the entire field of view, and then spread again in wavelength by the dispersion, these systems require very bright and expensive light sources, such as mercury arc lamps. Alignment, cost, and complexity are all well-known difficulties with perforated disc type confocal microscopes, which tend to be very expensive.
While Molesini used a refractive lens to provide the axial dispersion, Gross et al in U.S. Pat. No. 4,585,349, describe a chromatic confocal height measuring sensor that uses a holographic element to provide the axial dispersion. It includes a pinhole and a diffraction grating to detect the height signal. The holographic element serves the dual purpose of causing axial dispersion as well as focusing the light onto the object. A different holographic element is required for each desired spot size and measurement range. In addition, the difficulty of fabricating a well-corrected holographic element with large numerical aperture suggests that a small spot size would be difficult to achieve.
A holographic element is one type of diffractive lens. Another type is a zone plate. In “Diffractive Lenses for Chromatic Confocal Imaging,” Applied Optics, Vol. 36, No. 20, p. 4744, 1997 by Dobson et al, describes a zone plate with a positive refractive lens and a microscope objective lens to form a chromatic confocal system.
Baumann in U.S. Pat. No. 6,038,066 discloses an afocal dispersive system inserted in the infinite conjugate region of a confocal microscope. However, the considerations behind Baumann's design and the present invention are different. Baumann's design provides a small amount of predetermined axial dispersion. For example, the axial range of focus for a 200 nm wavelength band with a 20× microscope objective is about 25 μm. The present invention is designed for a much greater range of focus. For example, the axial range of focus for the present invention in the same condition is 200 μm. Baumann's design is well corrected for transverse chromatic aberrations, curvature, coma, astigmatism, and distortion. The off-axis aberrations are important in Baumann's design because it is intended for use with a scanning, imaging, confocal microscope, such as those that use a rotating Nipkow disc. However, the present invention is designed for a single point height measurement sensor, and so the off-axis aberrations are not a chief consideration.
The principles of confocal microscopes are described in “Confocal Scanning Optical Microscopy and Related Imaging Systems”, Academic Press, 1996 by Corle et al. A confocal microscope begins with a pinhole at the source, which is imaged onto the object. The objective lens images the object spot onto another, sometimes the same, pinhole before a detector. If the object is out of focus, then the re-imaged spot will be out of focus at the detector pinhole. As a consequence, the light power at the detector is maximum when the object is exactly in the focal plane of the objective, and when the object is out of focus, most of the light power is blocked by the detector pinhole. If the object is scanned axially through focus, the axial position where the detector power is maximum can be recorded, thereby measuring the surface height of the object. The final result is that a confocal microscope provides an intensity signal corresponding to the surface height of the object.
None of the above prior art addresses the measurement of a narrow, high aspect ratio trench. Neither does any of the prior art use separate wavelength bands to integrate a chromatic confocal height sensor with a traditional reflection microscope.